A magnetic resonance imaging system generally includes a main magnet, gradient coils (x, y and z), an RF amplifier, an RF transmit coil, and an RF receive coil. The main magnet polarizes protons in tissue in a human or animal subject in an examination region. The gradient coils localize and spatially encode the positions of the protons. The RF amplifier produces an RF signal which causes the RF transmit coil to transmit RF pulses that excite protons in the subject. The RF receive coil receives a Magnetic Resonance (MR) signal produced in response to the protons returning to the pre-excite state. The received MR signal is processed to generate an image.
The finite conductivity of anatomical tissue of the human or animal subject causes losses as the RF pulses are transmitted into the anatomical tissue. This energy loss (as resistive Ohmic losses) in the body tissue decreases the Q of the MRI coil, which necessitates increasing the power requirement to generate a given magnetic field. As the mass inside the RF transmit coil varies from subject to subject, so will the resistive losses and so will the Q of the coil. As such, the load impedance the RF amplifier sees may be different from a 50-Ohm load for certain subjects (e.g., as high as 4:1 Voltage Standing Wave Ratio (VSWR) impedance mismatch for a child subject).
A similar load mismatch situation occurs in a two-channel MRI system where an interaction between the two channels (the two MRI coils) takes place and feeds/couples back to the two RF amplifiers feeding the two channels. Unfortunately, the interaction between the two channels may require isolators/circulators to prevent the two RF amplifiers from interacting together and to protect each channel from the excessive reflected power, or load mismatch at the output of each RF channel. Isolators/circulators tend to be expensive and add components, mass, and cost to the RF amplifier and consequently, to the MRI system.
A load mismatch condition results in wasted reflected power back into the RF power amplifier, which is not used as part of the MRI coil excitation. In both of the above situations, the load impedance mismatch conditions can result in up to, e.g., 36% of reflected power into the RF amplifier. This reflected power into the amplifier will increase the power dissipated into the RF devices used in the amplifier, which will lower the RF amplifier Mean Time Between Failures (MTBF). Increasing the power to compensate for losses may increase the cost (e.g., increase the electric bill) of operating the system. The losses may also limit the maximum output power that could be achieved from the RF amplifier.